CUPERTINO, Calif. — Apple Inc. co-founder and CEO Steve Jobs is back at his office a few days a week after taking a 5 1/2-month medical leave and getting a new liver.

Jobs, 54, will work from home on days he doesn't work from Apple's Cupertino, Calif., headquarters, company spokesman Steve Dowling said Monday. Dowling did not say exactly when Jobs returned to the office.

The state of Jobs' health and the timing of his return have been watched closely by investors and the media, because few CEOs are considered as instrumental to their companies' success as Jobs has been to Apple. He is seen as the visionary behind Apple's popular iPod music players and the iPhone, which left far more experienced mobile phone makers scrambling to catch up with similar touchscreen devices.

The Apple chief was diagnosed with a rare form of pancreatic cancer called an islet cell neuroendocrine tumor. He had surgery in 2004 and announced then that he was cured.

Last year, Jobs' dramatic weight loss prompted new questions about his health, which Apple only intensified by saying in December that the CEO would not deliver the opening keynote at the upcoming Macworld conference.

In early January, Jobs said in a statement that he was suffering from an easily treated hormone imbalance, but he reversed course less than two weeks later, saying his medical condition was more complex than he initially thought. He announced he would take a leave of absence until the end of June.

Methodist University Hospital Transplant Institute in Memphis, Tenn., said last week that Jobs had received a liver transplant. Medical experts who were not involved in Jobs' treatment have told The Associated Press that cancer cells not removed in the original surgery could have spread to Jobs' liver.

The hospital would not say when the transplant took place, but in a statement said Jobs was recovering well and his prognosis is good.

Since Jobs returned to Apple in 1997 after a 12-hiatus, the company has expanded from a niche computer maker to become the top producer of portable media players and an increasingly important player in the cell phone business. Job's insistence on elegant design, and his ability to persuade consumers to spend more for it, has also given Apple's Mac computers a boost.

But under the direction of Apple's chief operating officer, Tim Cook, the company had continued to release well-received products during Jobs' leave, including updated laptops with lower entry-level prices, updated Mac software and a faster iPhone with many longed-for features. Apple sold more than a million of the new iPhone 3GS during its first three days on the market.

News and rumors have about Jobs' health sent Apple stock soaring and sinking, but the company has largely kept investors in the dark about the details of the CEO's condition and care. Federal rules around what information Apple must disclose to shareholders aren't specific on the matter of executive health unless the information would affect a reasonable investor's decision to buy or sell a stock.

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